Linux System Administration

Update: 04.03.2010 - Section 7.2 was created to clarify relation between Pointers and Arrays in C++

Introduction

This article is intended to all programing enthusiasts on all levels who do wish to understand pointers in C++ language. All code presented here is not a compiler specific and all examples will be written in plain ANSI C++. Debate about pointers can stretch for miles, and you would need to go really far to master it all. If you really want to run that far, this article gives you a clear understanding of fundamental concepts about pointers and prepares you for that journey. However, those who are new to C++ programming make sure that you are able to write and run your own C++ “hello world” program, and also it is recommended that you have a basic understanding of C++ functions and classes. If you need to refresh your knowledge about how to compile and run C++ program, use functions and classes, please read an appendix at the end of this document before you continue reading this article.

What is a Pointer?

Pointer is a variable that stores a memory address. OK, that is simple ! But, what is a memory address then? Every variable is located under unique location within a computer's memory and this unique location has its own unique address, the memory address. Normally, variables hold values such as 5 or “hello” and these values are stored under specific location within computer memory. However, pointer is a different beast, because it holds the memory address as its value and has an ability to “point” ( hence pointer ) to certain value within a memory, by use of its associated memory address.

Retrieving a Variable's Memory Address

OK, enough talking and let's get down to the pointer business. To retrieve a variable's memory address, we need to use address-of operator &.

The first line of the output contains an integer value 99 and on the second line, there is a memory address of myInt printed out. Please note that your output will be different.

Assigning a Variable's Memory Address to a Pointer

Before we can assign a memory address to a pointer, we need to declare one. Declaring a pointer in C++ is as simple as to declare any other variable with one single difference. Asterix symbol " * " needs to be add and located after variable type and before a variable name. One rule has to be followed when assigning memory address to a pointer: pointer type has to match with variable type it will point to. One exception is a pointer to void, which can handle different types of variables it will point to. To declare a pointer pMark of type unsigned short int a following syntax is to be used:

#include <iostream>

int main(){usingnamespace std;

// Declare and initialize a pointer.unsignedshortint * pPointer = 0;// Declare an integer variable and initialize it with 35698unsignedshortint twoInt = 35698;// Declare an integer variable and initialize it with 77unsignedshortint oneInt = 77;// Use address-of operator & to assign a memory address of twoInt to a pointer pPointer = &twoInt;// Pointer pPointer now holds a memory address of twoInt

The diagram above is a high level visual abstraction of how are variables stored within a computer memory. Pointer pPointer starts at memory address 0xbff43314 and takes 4 bytes. Pointer pPointer holds as a value a memory address of a short int twoInt ( 2 bytes ) which is 0xbff4331a. This address is stored as a binary data within a pointer's memory space allocation. Therefore, dereferencing a pointer with a memory address 0xbff4331a will indirectly access a value of twoInt which is in this case a positive integer 36698.

Here is a small example of C++ code on how to read a characters from a file as well as to count the number lines of any particular file consist of. The code will check for "\n" the "new line character" and increase the number of lines stored in number_of_lines integer variable. Every iteration will also print single character including "\n" to an output. First create a file called my-input-file.txt which will contain some text. For example:

Here is a small program to clalculate Fibonacci numbers using c++ language. No tampering with the code needed ( only iprovements ! ) just copy and compile with g++ FibonacciNumber.cpp -o FibonacciNumber and run ./FibonacciNumber